Single side band diversity radio receiving system



Oct. 29, 1940. A. A. oswALD SINGLE SIDE BAND DIVERSITY RADIO RECEIVINGSYSTEM Filed Aug. 5l, 1939 Patented Oct. 29, 1940 NETE T E S SINGLE SIDEBAND DIVERSIT'Y RADIO RE- CEIVING SYSTEM Arthur A. Oswald, Maplewood, N.J., assignor to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Application 6 Claims.

This invention relates to radio communication systems, and'more'particularly to methods and means for reducing fading effects andmaintaining a favorable signal-to-noise ratio in a diversity receptionsystem. The present application relates to a specific modification ofthe invention disclosed and claimed in the copending application SerialNo. 291,481, led August 23, 1939, of

F. A. Polkinghorn, assigned to the same assignee to which the presentapplication is assigned.

In a diversity system the same transmitted signal is simultaneouslyreceived over a plurality of paths through space from the signal sourceat the transmitting station. The amplitude of received 'signal over thevarious paths usually varies as between the different paths with changesin the condition of the space transmission medium, so that as theamplitude of the signal received over one path increases, that overanother or other of the paths may diminish. These amplitude variationsa-re generally at random and are continually taking place.

It is customary in certain types of diversity systems either to providemeans for automatically switching the signal receiving apparatus at thereceiving station so as tokeep it connected with the path over which thesignal of the greatest amplitude at any particular time is arriving, or,alternatively, to permit the signal receiving apparatus at all times tobe connected with all of the paths and to automatically control theenergy contributions to the signal receiver by the various paths inproportion to the varying amplitudes of the signals being received overthem. It is toward diversity operation of the latter sort that thepresent invention is particularly directed.

In the double side band typeof radio transmission the carrier upon whichthe side bands are basedis usually transmitted with the side bands atsufficient amplitude to provide for the proper demodulation of the sidebands in the final de- -tector at the receiving station. When generalfading takes place, the carrier fades with the side bands; and as thenoise introduced at the receiving station input of the fading path istransmitted to the receiving apparatus as a modulation of the carrierfrequency, the same fading of the carrier and its side bands whichreduces the signal amplitude also reduces the noise amplitude.

In the single side band type of radio transmission it has been thepractice either to eliminate the carrier entirely, or to transmit it a1;a much reduced energy level. When no carrier is transmitted, a suitabledemodulating carrier of the proper frequency is generated and suppliedat the August 31,

1939, serial No. 292,777

(o1. 25o-2o) receiving station. When carrier is transmittedV at areduced energy level, it is customary either to utilize it at thereceiving station to control the frequency of a local oscillator thatsupplies the frequency for final demodulation, or, after reconditioningand amplifying at the receiving station, to utilize it at the properfrequency directly to effect the demodulation of the signal side bands.In either case the locally generated demodulating frequency, or thedemodulatingl fre- .'1'0 quency controlled by the original' carrier, orthe reconditioned and amplified original carrier itself at the properfrequency for demodulation has in some cases been supplied to the finaldetectors at a fixed amplitude that does not vary with the variation ofcarrier amplitude that may take place as a result of the presence orabsence of fading over the space path.

Such practice is not favorable toA the best operation of a diversitysystem in which the plurality of paths over Which'the signal is receivedare at all times connected with the signal receiving apparatus at thereceiving station. This is because the fading over any path'whichdiminishes the amplitude of the signal does not diminish the amplitudeof the noise that enters the receivingl system byrway of the receiverinput of that path. `The constant amplitude of the locally suppliedcarrier, or of the'recondtioned received carrier, causes the relativenoise output` of that path to remain constant even though the signalcontributed by the path has faded. Therefore all Of-the'paths contributenoise to the commonv signal receiving apparatus, while only those ofthem in which fading has not occurred contribute signal at a favorablesignal-to-noise ratio. The result is that, in the absence of somecorrective measure, the over-all signal-to-noise ratio of such a systemis adverselyaffected.

In the diversity system of the present invention this eect is overcomebythe employment of a method and means for controlling the amplitudes ofthe signal and accompanying noise passing by Way of each of thediversity branches to the common signal receiver in accordance with theamplitude of a selected portion of the energy received from thetransmitting station over the corresponding space path. Specically, inthe preferred embodiment, when the signal and selected cor'it'rollingportion, which may for example be the accompanying carri-er or anaccompanying pilot frequency, of any path fade, the gain in the signaland noise transmitting portions of the corresponding diversity branch iscaused likewise to bediminished. Thus, the output of anyone or 55 moreof the diversity branches upon which the signal is fading is reduced inaccordance with the amount of fading, and a more favorable overallsignal-to-noise ratio is maintained in the common signal receiver intowhich all of the branches feed.

The manner in which this result is accomplished will be explained byreference tothe accompanying drawing which represents schematically asingle side-band diversity receiving system consisting, as illustrated,of two radio reception branches, each arranged to receive radio signalsarriving from the same signal source over different space paths, convertthe signals to audio frequencies and transmit them to a common signalreceiving device.

For the sake of simplicity, communication of the radio receivingapparatus with the different space paths is represented as being by Wayof the antennas AI and A2, which may be of any suitable type. Theseantennas feed over radio reception branches I and 2, respectively, tothe common signal receiver I0. Only two antennas with their associatedradio receivers are shown, but it will be understood that any greaternumber, usually three in a space diversity system, may be used. Theelements included in each radio reception branch are of identically thesame type as those included in the other branch; and correspondingelements are designated by the same reference numbers, excepting that aprime is aflixed to the designating numerals of the second branch. Thedescription of the arrangement and operation of the elements in onebranch may therefore be understood to apply to the operation andarrangement of the elements in the other branch.

The energy received by the antenna passes to high frequency amplifierand first detector elements II. In the first detector the receivedenergy is modulated by a high` frequency wave supplied by the beatoscillator 20 and the desired products of modulation are amplified in atwostageintermediate frequency amplifier I2. The above-mentionedVelements may be regarded as 1 constituting the radio receiver of thereception branch. 'Ihe intermediatefrequency amplifier I2 operates intoa branchedzcircuit, one branch oi which includes an isolation amplifierI3 and a lsecond detector I4, and the other branch of which selects anarrow frequency band including the carrier that passes through `thebandpass crystal filter I5 to a rectifylng device which in the presentinstance consists of the two detectors I6 and I 'I.

, To the output of the second detector I4 there is supplied a localcarrier frequency from the local carrier oscillator 30, andV the audiofrequency products of modulation of the local carrier and the signalintermediate frequencies feed into the input of audio frequencyamplifier I8 by way of transformer 2I The degree of ampliiication of theaudio frequencies in amplifier I8 is controlled by the potential acrossthe resistance I9 included in the output circuit of the detector I6,this potential beingapplied in opposition to the normal biasinglpotential in the input circuit of the amplifier I8. The amplified audiofrequencies of branch I, together with those of branch 2, are theninductively communicated by way of transformers 23 and 23',respectively, to a circuit' in which they are combined, furtheramplified if desired, and passed to the common signal receiver I 6.

The detector tubes I7 and I'I of branchI and branch 2, respectively,control the potential drop across the common resistance 30, and thevariable potential thus developed is applied to the amplifiers I2 and I2of branch I and branch 2, respectively, to give the usual diversitycommon automatic volume control to the system.

In the operation of the system it is assumed that the distant station istransmitting a radio frequency carrier of relatively small amplitude,and a single side band resulting from the modulation of the carrierfrequency with the signal frequency. The radio receiver elements II andII' of the two radio reception branches I and 2 are tuned to select thecarrier and its side band,

`and the beat oscillator 20 is so adjusted as to produce, by modulationwith the incoming wave, an intermediate frequency wave of the desiredfrequency, for instance, 400 kilocycles. A portion of this ampliedintermediate frequency, as has been described, passes into the branchcircuit including the band-pass crystal iilter I5. This filter may bedesigned to pass a frequency band the width of which is suflicient toaccommodate the carrier at its intermediate frequency value with suchfrequency variation as may occur on either side of this value as aresult of imperfect frequency stability in the carrier oscillator at thetransmitting station and in the beat oscillator 20 at the receivingstation. For a moderate degree of frequency instability in theseoscillators the band passed by the iilter I5 may be 200 cycles wide, ornarrower to the extent that the oscillator stability warrants. Afterpassing through filter I5 the narrow band including the carrier at itsintermediate frequency value passes to the input circuits of detectorsI6 and I'I.

In the output of detector I6 there is produced a direct current theamplitude of which Varies with the variations in the amplitude of thecarrier received over the corresponding branch. Any fading of thecarrier and its associated side band at the receiving antenna of thecorresponding branch results in a reduction in current flowing throughthe resistance I9 in the output circuit of detector I6 and acorresponding potential reduction across the resistance.

The bias in the input circuit of the audio frequency amplifier tube I8is determined by the potential of the biasing battery 22 and thepotential drop across the resistance I9. This bias is so adjusted thatwith nocarrier input to detector I6, amplifier I8 substantiallysuppresses transmission to the common signal circuit. With a normalcarrier input to detector I6 the in- 'g creased potential acrossresistance I9, in opposition to the potential of biasing battery 22,decreases the negative bias in the input circuit'of audio frequencyamplifier I8 to produce normal gain in the transmission of the signal ofthis branch to the signal receiver. Thus, when the fading over any pathis such as to produce an unfavorable signal-to-noise ratio, the gain inthe corresponding branch of the system is automatically reduced inproportion to the reduction of the radio frequency input to thecorresponding antenna, and the contribution of that branch to the commonsignal receiver circuit is proportionately diminished or substantiallysuppressed.

An indicated in the drawing, the amplifier I3 in the signal sub-branchof each of the branches I and 2 is of the screen gridV type forpreventing the local carrier oscillations which are fed into the platecircuit of the second detector I4 from 75 feeding back through theychannel and appearing in the other detector circuits.

The diversity common automatic volume control which is secured byemploying the voltage drop across the resistance 40 common to the platecircuits of the detectors I1 and I'I' to control the grid bias of theamplifiers of several branches simultaneously and equally, is Well knownin the art and needs no specific description. It ensures that the radiofrequency outputs of the respective receivers of the several branchesshall have the same ,relative signal strengths as the radio frequencyvinputs to the receivers. Y

If it be assumed that, from aV condition ofv equality, the radiofrequency energy received upon antenna AI and transmitted through radioreception branch I increases, while that received upon antenna A2 andtransmitted through radio reception branch 2 diminishes, the commonautomatic volume control eiiected by detectors Il and Il and theindividualtransmission gain control effected by detectors I6 and IE willoperate in such a way as to tend to magnify the contribution of branch Iwhich has the more favorable signal-to-noise ratio and to reduce thecontribution of branch 2 which has the less favorable signal-to-noiseratio. The increasing negative bias on the amplifier tubes of bothbranches resulting from the .increasing radio frequency input fromantenna AI stabilizes the volume contribution of branch I at apredetermined value and reduces the relative contribution of branch 2 inwhich the signal is fading. At the same time detector I6 of branch Ioperates to reduce the negative bias in the input circuit of audiofrequency amplifier I8 to permit a proportionately greater signaltrans-` mission through this branch to the common lsignal receiver;While detector IS' of branch 2,

energy received over the corresponding branch.

The result is that the signalreceived in the common signal receiver Il)is principally that re-l ceived over the path of the diversity systemwhich has the most favorable signal-to-noise ratio,

while the noise that Would otherwise enter they common receiver over thepath or paths of reclucedsignal energy is diminished or completelysuppressed by the device which reduces the gain of the transmission pathin accordance with the reduction in the received radio frequency energy.

In a properly proportioned system in accordance` with the presentinvention the signal volume delivered to signal receiver I is heldsubstantially constant.

\ The interposition of a rectifled-carrier-operated gain controllingdevice, such as the audio frequency amplifier I8, in the path which thesignals and noise must traverse to reach the common signal receiverII'I, further serves the useful function of Vexcluding from the commonsignal circuit modulation products due to the intermodulation of noisecomponents with each other, which intermodulation may take place even inthe absence of the locally applied modulating carrier frequency.Whatever the nature of the noise and signal demodulation in the seconddetector may be, the transmission of the signal and noise componentsthrough that part of the signal sub-'branch that includes the second'detector and audio frequency amplifier is controlled in amplitudeinaccordance with the amplitude of the rectified energy received from thetransmitting station.

If it `should berdesired to effect a greater degree of frequencystability, thisl maybe done in the manner disclosed in Patent 2,041,855to R. S.y That is, the filteredk Ohl, issued May 26, 1936. carrierreceived from the transmitting station may bereconditioned andthen usedfo-r obtaining automatic frequency control of the beating oscillator 2l)and synchronization of the local carrier oscillator 30. Or, if desired,in accordance with the disclosure of the Ohl patent, the filteredcarrier after being reconditioned and amplified may consitute the sourceof locally applied carrier oscillations in place of the local carrieroscillator 30. As ldisclosed -in the Ohl patent, the reconditionedcarrier is supplied for demodulation at substantially constant amplituderegardless of fading in transmission, and therefore, if applied in thepresent system in substitution of the carrier oscillations generatedlocally by oscillator 3B, thesystern is susceptible of and requires thesame automatic gain control in accordance with the amplitude of therectified received carrier energy of the corresponding branch that isutilized in the present invention.

The specific disclosure herein is that of a diversity systemin which thesame signal is simultaneously received over a plurality of paths by Wayof separate antennas. It will be understood that the invention is alsoapplicable to an angle diversity systemt in which the same signal issimultaneously received at different vertical angles by Way of a singlearray of spaced antenna units, andis demodulated at each angle byreconditio-ned carrier arriving at that angle, or by locally generatedcarrier synchronized by the received carrier of the corresponding path.

It will also be understood that the transmission gain control of eachreception branch in accordance with the amplitude of the rectifiedenergy received from the transmitting station over that branch may,Within the scope of the invention, be alternatively applied at otherpoints in the signal conveying portion of the sub-branch, as at theinput of the isolation amplifier I3.

In the foregoing the energy received over each branch and selected andrectified to control the signal transmission gain of that branch, hasbeen specifically described as that of the transmitted carrierfrequency. It will be understood that in systems where the carrier isentirely suppressed and a pilot frequency is transmitted in its place,as a side-band of the suppressed carrier, this pilot frequency may beselected and rectified and used to control the signal gain of thecorresponding branch in the same manner as has been described for thecarrier.

What is claimed is:

1. In a diversity radio receiving system, a plurality of radio receiversarranged to receive radio signals from the same'signal source bydifferent paths, means for selecting and rectifying a portion of theenergy received from the transmitting station by each receiver, meansfor combining the rectified currents, means for controlling theampliiication of all the receivers in accordance with the combinedrectified currents so that the radio frequency outputs of the respectivereceivers have the same relative signal strengths as the inputs thereto,a signal detector for each receiver, said detectors having their inputsconnected to the radio frequency outputs of the respective receivers,means for supplying carrier oscillations to said detectors, an amplifierfor each receiver connected in the signal conveying circuit of thecorresponding detector, an audio frequency circuit common to saidamplifiers and signal detectors of all of said receivers, and meansresponsive to said individual rectified currents for so controlling theamplification of the respective amplifiers and detectors as to maintainthe audio signal contribution of each receiver tosaid common circuitproportional to the respective received radio signal strengths.

2. In a reduced carrier diversity radio receiving system comprising aplurality of radio reception branches each including a radio receiverand each connected with a common signal receiver, means operativelycoupled to each branch for selecting and rectifying a portion of thesignal carrier, means under the control of the combined rectifiedcarrier currents of all the branches for similarly controlling theamplification of the radio receivers of all of the branches so thattheradio frequency outputs of the respective receivers have the samerelative signal strengths as the inputs thereto, a signal detector foreach branch, a substantially constant amplitude source of carrieroscillations at the receiving station connected to the detector inputsof the various branches, an amplifier for each branch included in thesignal conveying circuit of the corresponding detector, and meansresponsive to the amplitude variations of the rectied carrier current ofeach branch for controlling the gain of said amplifier of thecorrespond- "l ing branch.

3. In a diversity radio receiving system comprising a plurality of radioreception branches each including a radio receiver and each connectedwith a common signal receiver, means operatively coupled to each branchfor selecting and rectifying a portion of the energy received from thetransmitting station by each receiver, means under the control of thecombined rectied currents of all the'branches for similarly controllingthe amplification of the radio receivers of all of the branches so thatthe radio frequency outputs of the respective receivers having the samerelative signal strengths as the inputs thereto, a signal detector foreach branch, a substantially constant amplitude source of carrieroscillations at the receiving station connected to the detector inputsof the various branches,` an amplifier for each branch included in, thesignal conveying circuit of the corresponding detector, and meansresponsive to the amplitude variations of the rectified carrier currentof each branch for controlling the gain of that portion of thecorresponding branch which includes said signal detector and saidamplifier.

4. In a single side band diversity radio receiving 'system comprising aplurality of radio reception branches each connected with a commonsignal receiver, means operatively coupled to each branch for selectingand rectifying a portion of the energy receivedfromy the transmittingstation by Way of the corresponding branch, a signal detector foreachbranch, a substantially constant amplitude source of carrieroscillations connected to the detectors of the various branches, anaudio frequency amplifier for each branch included between the detectoroutput of the corresponding branch and said common signal receiver, andmeans responsive to the amplitude variations of the rectified energy ofeach branch for controlling the gain of said audio frequency amplifierof the corresponding branch.

5. In a diversity radio receiving system comprising a plurality of radioreception branches connected with a common signal receiver and eachincluding a radio receiver, means operatively coupled to each branch forselecting and rectifying a portion of the energy received from thetransmitting station by way of the corresponding branch, means operatedby the combined rectilied energy derived from all of the branches forsimilarly controlling the amplification of the radio receivers of all ofthe branches so that the radio frequency outputs of the respectivereceivers have the same relative signal strengths as the inputs thereto,a signal detector for each branch, a substantially constant amplitudesource of carrier oscillations at the receiving station connected to thedetectors of the various branches, an audio frequency amplifier for eachbranch included in the signal detector output of the correspondingbranch, and means responsive to the amplitude variations of therectified current of each branch for controlling the gain of said audiofrequency amplifier of the corresponding branch.

6. The method of maintaining a favorable signal-to-noise ratio in adiversity radio receiving system utilizing means for receiving indifferent radio reception branches the radio signals arriving from thesame signal source over different space paths, applying equalamplification to the energy received in each branch, separatelyselectind and rectifying a portion of the energy received over eachbranch, and separately demodulating the signal energy component of eachbranch by carrier oscillations supplied from a substantially constantamplitude local source, which consists in separately amplifying thedemodulated signal energy of each branch, utilizing the rectifiedportion of the energy of each branch to control in accordance With itsown amplitude the degree of amplification of the demodulated signalcomponent of the corresponding branch, and combining and utilizing thedemodulated sig- :ual components of all the branches.

AR'IHUR. A. OSWALD.

